1. Field of the Invention
The present invention relates to an electrolytic capacitor tightly sealed within a glass envelope, including a sintered body of electrochemical valve metal coated with a dielectric oxide film, a semiconductor layer arranged thereon, and a conducting layer on the semiconductor layer.
2. Description of the Prior Art
The use of electrical capacitors sealed in evacuated glass envelopes with terminals which pass through the envelope, is known. As shown in U.S. Pat. No. 2,283,723, it is also known to make a glass encapsulated electrolytic capacitor having an anode of electrochemical valve metal. Known electrolytic capacitors have included a sintered body of electrochemical valve metal, a dielectric oxide film, with a semiconductor layer arranged thereon and a conducting layer on the semiconductor layer, which are sealed in a glass casing.
There have been no difficulties in sealing electrostatic capacitors in a glass casing and it is known which types of electric leads are best suited for sealing to glass. In the case of liquid electrolytic capacitors, there was a problem of tightly sealing the glass casing without interference from the liquid electrolyte. This problem, however, has been solved and the sealing of liquids in glass capsules or ampoules is also known.
Special problems, however, arise when sealing electrolytic capacitors having a semiconductor layer within a glass casing. As is well known, the semiconductor layer often consists of an oxide film, such as manganese dioxide which is mechanically sensitive and difficult to establish contact thereto. The contacting of this semiconductor manganese dioxide layer, is usually carried out in such a way that a carbon or graphite layer is deposited onto the manganese dioxide layer, with a further conducting layer in the form of a metal layer such as silver being deposited onto this graphite layer. However, the metal or conducting silver layer does not adhere very well to the carbon or graphite layer and the latter, in turn, does not adhere very well to the manganese dioxide layer therebeneath. Due to this relatively poor mechanical connection, resistance variations occur in these layers or on the boundary surfaces or interfaces, which may cause a considerable variation in the electrical properties of the capacitor.
During the sealing of an electrolytic capacitor into a glass casing, difficulties are likely to be encountered with respect to establishing the electrical connection to the semiconductor layer or the conducting layer arranged thereon, or between the conducting layers and the lead wires sealed into the glass casing. It may be possible to solder the conducting layer to the sealed-in cathode lead by inserting a piece of solder metal into the glass casing sealed on one side, with the capacitor body being inserted thereafter and the solder metal being heated to cause the melting thereof and effect soldering between the capacitor body and the cathode lead. However, it is very difficult to establish a solder connection inside the narrow glass casing. A good solder connection requires the addition of a suitable flux agent. This, however, encounters difficulties in the final sealing of the glass casing, due to the development of gases from remainders of the flux agent. The flux agent also has an unfavorable influence upon the impedance of the capacitor. Variations of the impedance and of the resonant frequency also occur in the case of soldered contacts between the capacitor body and the cathode lead in cases where the capacitor, subsequent to the sealing of the glass casing, is subjected to tempering at a higher temperature for the purpose of stabilizing its electrical properties. Accordingly, soldered joints have been found unsuitable for use in this particular case.
Previously known outer conducting metal layers of capacitors were sprayed on the semiconductor layer. These layers, however, were generally connected to a cathode lead by way of soldering, or consisted of a metal or an alloy having a relatively high melting point and no porous structure. It has also been known to deposit a layer by spraying small plate-shaped particles of copper. This, however, will not result a porous metal body because the small plate-shaped particles are placed closely on top of each other. Due to the high melting point and the relatively great hardness of copper, such a layer is unsuitable for absorbing electrical contact pressure.